The present invention relates to the plasticizing and extrusion of thermoplastic resins in injection molding applications and in particular to a static mixer for generating a substantially homogeneous melt stream.
Static mixers have been used for many years as a means of generating a homogeneous melt stream from a screw type resin plasticizer. One early type of mixer is known as the "Kenics" mixer. In this type of mixer, the incoming melt stream is split into two paths. reoriented 90.degree. and discharged into a chamber prior to entering a second stage of splitting and reorientation. A "Kenics"-type mixer is illustrated in the paper "Post Extruder Mixing and Thermal Homogenization of Plastic Melt" by S. J. Chen and in U.S. Pat. Nos. 3,386,992, 3,664,638 and 3,704,006. The primary disadvantage associated with this type of mixer is the considerable length of mixing required in order to obtain a homogeneous mix.
A development arising from the "Kenics" principle is the "Komax" mixer. This type of mixer is characterized by a central boss, six relatively thick radial arms curved to define a plurality of circular passageways, and intermediate curved members for dividing each passageway. The curved members rotate through approximately 180.degree. from a radial direction to a circumferential direction. The primary disadvantages associated with this mixer are the significant pressure drops encountered during use due to the relatively thick, non-uniformly dimensioned radial arms and the melt hang-up and degradation due to the curved surfaces defining the flow passageways.
Still other mixers based on the "Kenics" principle are the Dow/Ross mixer and the Koch mixer. One type of Dow/Ross mixer is an interfacial surface generator mixer similar to those exemplified by U.S. Pat. Nos. 3,404,869 and 3,583,678. As shown in the U.S. Pat. No. 3,404,869 the mixer may consist of a number of tetrahedral, rectangular or circular chambers having two or more inlets and two or more non-coplanar outlets. A plurality of separate passageways, as shown in the U.S. Pat. No. 3,583,678, may be used to connect the inlet and outlet ends with the passageways opening on the inlet end along a first line lying approximately through the center of the inlet end and opening on the outlet end along a second line lying approximately through the center of the outlet end and being essentially normal to the first line.
A second type of Dow/Ross mixer is shown in the brochure "Ross Motionless Mixers" by Ross Engineering Inc. This type of mixer consists of a series of semi-ellipical plates discriminately positioned in a tubular housing. A single element consists of two plates perpendicular to each other. Hereagain, the mixing operation is based on splitting and then diverting input streams.
"Koch"-type mixers are illustrated in U.S. Pat. Nos. 3,785,620 and 3,871,624 as well as in the brochure "Koch Melt Blender" by Koch Engineering Company Inc. The mixing apparatus shown in the U.S. Pat. No. 3,785,620 uses a plurality of packing elements in the flow path for mixing the media together by longitudinal and transverse mixing. Each packing element includes a plurality of lamellas in contact with each other. Each lamella has corrugations thereon with the corrugations of sequentially arranged lamellas being oriented in different directions. In addition, corrugations of at least one of two sequentially arranged lamellas relative to the direction of media flow are disposed at an angle to the axis of the apparatus. Adjacent packing elements abut and are angularly offset to each other about the axis to enhance mixing. The mixing apparatus shown in the U.S. Pat. No. 3,871,624 has a plurality of mixing elements. Each mixing element includes a plurality of layers which contact each other and bound flow channels. The longitudinal axes of the flow channels in each layer extend substantially parallel to one another at least in groups while the longitudinal axes of flow channels in adjacent layers are inclined relative to one another. In a first embodiment, the mixing elements are layers of flat thin plates to which flat thin grid elements disposed at an angle are connected. In another embodiment, each layer in a mixing element comprises a tube bank.
Most static mixers subdivide and recombine the media flow several times during its passage through the mixer. The degree of efficiency with which this is done varies considerably. The main parameters to be considered are pressure drop, amount of mixing, amount of melt hang-up in trapped corners and the cost of producing the mixer. The Dow/Ross type of mixer encounters a large pressure drop during one since the melt passes through relatively small diameter holes. The Koch and Komax type mixers also encounter moderate pressure drops during use but more importantly have many places for the melt to hang-up and degrade. This is because the faces of their mixer elements have significant areas perpendicular to the melt flow. These perpendicular areas create dead spots where the melt can collect and remain stationary for an extended period of time. This causes the melt to degrade. Finally, the Dow/Ross and Komax mixers are very expensive to produce while the Koch mixer tends to collapse under high pressure applications.
Still another type of mixer is illustrated in U.S. Pat. No. 3,989,434. This type of mixer takes the form of a torpedo having a plurality of alternatively isolated and intersecting flow paths for repeatedly dividing and recombining material flow streams. The paths are formed as right and left-hand open helical channels on the circumference of a body with helices of one-hand having a pitch different from that of the other such that flow paths intersect in a pattern which is bilaterally asymmetrical with respect to an axial net flow direction. Torpedo type mixers such as this are used in injection molding machines of the ram feed type where the inherent agitating action of a screw feed is not present.
All the above-noted disadvantages are believed to be successfully overcome by the static mixer of the present invention.
It is an object of the present invention to provide a static mixer having superior mixing efficiency.
It is a further object of the present invention to provide a static mixer as above having relatively low pressure drops associated therewith.
It s still a further object of the present invention to provide a static mixer as above which substantially reduces and virtually eliminates the incidence of melt hang-up and degradation.
It is yet a further object of the present invention to provide a static mixer as above which is relatively compact and inexpensive to manufacture.
These and other objects and advantages will become more apparent from the following description and drawings wherein like reference numerals depict like elements.